Answer:
The issue of slavery gradually shook the foundations of the two-party system of the Whigs Democrats. Its noticeable aggravation occurred as a result of the adoption in May 1854 of a bill on the incorporation into the United States of the territories of Kansas and Nebraska, located north of 36°30, the border of slavery, established by the Missouri compromise of 1820. The initiator of the bill was the Democratic Party. It was based on the doctrine of "sovereignty of the settlers", according to which the status of slavery in the new states was to be determined by the population.
The doctrine of the "sovereignty of the settlers" that appeared in the political arsenal of the democrats in the late 40s, is difficult to be give an unambiguous assessment. Putting it forward, the leadership of the Democratic Party was looking for ways to ensure internal political stability and achieve a new compromise between the North and the South. On the other hand, the initiators of the bill assured the population of the North that the flow of settler farmers would undoubtedly exceed the influx of slave owners into new territories, which guarantees the future accession of these territories to the Union as free states. However, the Democratic leadership’s calculations for attenuation of disputes about slavery did not implement.
The center of gravity for all opponents of slavery was the new Republican Party. The Republicans entered the political arena with a program of radical reconstruction of the country's socio-economic structure through territorial restrictions on the institution of slavery. During these years, many moderately opposed opponents of the spread of slavery in Western territories feared the radicalism of the Republican Party.
Moderate positions on the issue of slavery determined the election of Lincoln as a compromise Republican presidential candidate in the 1860 election. Lincoln, thanks in large part to the split in the Democratic Party, which nominated two candidates, managed to get ahead of his rivals in the elections and become president of the United States. The first Republican president won the election, mainly due to support from the North.
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